Chitosan (also referred to as poly-(1→4)-β-D-glucosamine) is a biopolymer with many uses in the pharmaceutical, medical device, food and water treatment industries. For example, chitosan may be used as a delivery vehicle for a therapeutically active agent, in a synthetic bone graft material (e.g., a mixture of chitosan and hydroxyapatite), in spinal fusions, and in various wound healing applications such as hemostatic bandages. Chitosan has become an important hemostatic agent candidate since defibrinated blood, heparinized blood, and washed red cells all can form a coagulum in contact with chitosan. Hemostatic properties of chitosan depend on a novel hemostatic mechanism in addition to the clot formed by activation of fibrinogen and platelets. In addition, the strong tissue adhesive properties of chitosan increase its hemostatic efficacy. Other researchers have used chitosan for hemostasis in animals, for example, for vascular grafts in dogs, lingual hemostasis in rabbits, and topical hemostasis for diffuse capillary bleeding in animal brains. However for clinical hemostasis in humans, an important prerequisite is to reduce the immunological responses induced by the impurities in commercially available chitosan. Chitosan has also been marketed for use in consumer food products or nutritional supplements that are said to reduce fat and cholesterol.
Chitosan and its precursor, chitin, are typically prepared from waste shells of crustaceans, particularly decapod crustaceans such as crab, shrimp, crawfish, krill, lobster, squid and prawn. The conventional process for producing chitin and chitosan from crustacean shells involves grinding crustacean shells and treating the ground shells with a dilute base (e.g., sodium hydroxide) and heat to remove protein and lipids (deproteinization). Calcium carbonate is removed by extraction with a dilute acid (e.g., hydrochloric acid) at room temperature (demineralization). Following deproteinization and demineralization, the resulting product is predominantly chitin. An optional decolorization step may be used to bleach the chitin, for example, extraction with ethanol and ether, or bleaching with sodium hypochlorite. Removal of acetyl groups from the chitin polymer (deacetylation) produces chitosan; deacetylation is usually performed by reacting chitin with concentrated sodium hydroxide or potassium hydroxide and heat. The deacetylation process does not remove any contaminants existing in the chitin starting material. Thus, impurity removal for chitosan only occurs during production of the chitin precursor. Chitosan is not a single, definite chemical entity since it varies in composition depending on the crustacean species used for the starting material and the particular preparation method used.
Reducing or substantially removing impurities from chitosan that can cause immunological reactions is critical for chitosan intended for use as a biocompatible and biodegradable material in medical applications. However, purifying chitosan is very difficult since chitosan in solution is a highly viscous material. Producing highly pure, medical grade chitosan via the above-described conventional techniques is very expensive since such techniques typically require costly instrumentation such as autoclaves, ultrafiltration, and molecular sieves. The availability of less expensive medical grade chitosan should expand and accelerate its use in biomedical applications.